Metastatic Renal Cell Carcinoma: The Importance of Immunohistochemistry in Differential Diagnosis

INTRODUCTION: Clear cell carcinoma accounts for 75% of all types of renal neoplasms. Approximately one third presents with metastatic disease at diagnosis. Immunohistochemical studies play a significant diagnostic role. CASE REPORT: We report the case of a 48-year-old heavy smoker who presented with productive cough and progressive dyspnea. The study revealed a renal mass and lung alterations compatible with primary tumor of the lung. The patient underwent a right complete nephrectomy. The anatomopathological exam showed clear cell renal carcinoma (pT1bN0Mx). After transthoracic needle aspiration biopsy, the clinical diagnosis was stage IV adenocarcinoma of the lung. Initially, the patient received one cycle of chemotherapy (cisplatin/pemetrexed). Two weeks later, the immunohistochemistry tests revealed a secondary lesion with probable renal origin. Chemotherapy was stopped and the patient was started on sunitinib treatment. After two cycles the disease progressed. A second-line treatment with everolimus was proposed; however, the patient died 2 weeks later due to terminal respiratory insufficiency. DISCUSSION: Clear cell renal cell carcinoma remains one of the great mimickers in pathology. Immunohistochemistry is a valuable tool in the differential diagnosis of lung carcinomas. With the help of thyroid transcription factor 1, it is possible to distinguish a primary lung tumor from a metastasis with a reasonable degree of certainty. The present case report illustrates the challenge of making a definitive and adequate diagnosis. The immunohistochemistry added information that changed the whole treatment strategy. For the best treatment approach, it is fundamental that clinicians await all possible test results, before establishing a treatment plan.